make cache path control slightly more convenient

[MTCam]

This adds centralized setting of where mirgecom caches will go for Lassen, and overrides the default node-local system default setting of CUDA_CACHE_PATH.

User sets MIRGE_CACHE_ROOT to the path to locate all disk caches.
Fine grained control is still available as before.

Co-authored-by: Mike Campbell mtcampbe@illinois.eduQuestions for the review:

• [ ] Is the scope and purpose of the PR clear?
  • [ ] The PR should have a description.
  • [ ] The PR should have a guide if needed (e.g., an ordering).
• [ ] Is every top-level method and class documented? Are things that should be documented actually so?
• [ ] Is the interface understandable? (I.e. can someone figure out what stuff does?) Is it well-defined?
• [ ] Does the implementation do what the docstring claims?
• [ ] Is everything that is implemented covered by tests?
• [ ] Do you see any immediate risks or performance disadvantages with the design? Example: what do interface normals attach to?